1. Field of the Invention
The present invention relates to fuel cells and, more particularly, to methods of operating fuel cells having closed fuel supply systems.
2. Description of the Related Art
Electrochemical fuel cells convert fuel and oxidant to electricity. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”) which comprises an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. In operation the electrodes are electrically coupled to provide a circuit for conducting electrons between the electrodes through an external circuit. Typically, a number of MEAs are serially coupled electrically to form a fuel cell stack having a desired power output.
In typical fuel cells, the MEA is disposed between two electrically conductive fluid flow field plates or separator plates. Fluid flow field plates have at least one flow passage formed in at least one of the major planar surfaces thereof. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels or passages for the fuel and oxidant to the respective anode and cathode surfaces, and provide passages for the removal of reaction products, such as water, formed during operation of the cell.
Certain fuel cells are designed to operate in a closed mode on one or both reactants. Closed reactant supply systems include dead-ended configurations in which a reactant flow passage is generally closed, as well as systems employing closed-loop recirculation of the reactant exhaust stream from the fuel cell outlet to the fuel cell inlet and though the fuel cell with the addition of fresh reactant. In these situations the reactant used on the closed side is generally substantially pure. Typically a purge valve (which is normally closed in closed system operation) is provided somewhere in the reactant flow passage for periodic venting of accumulations of non-reactive components, which can build up in the reactant passages in closed system operation. In conventional fuel cell purge systems the purge valve is opened from time to time, for example, manually or at regular fixed time intervals. Alternatively a purge is triggered, for example, when the voltage or electrical output of one or more cells in a stack falls below a predetermined threshold value (see, for example, GB Patent No. 1 223 941), or when there is a predetermined decrease in electrical power output (see, for example, U.S. Pat. No. 3,553,026), or after the fuel cell has expended a preselected number of ampere-hours (see, for example, U.S. Pat. No. 3,697,325). The reactant flow path through the fuel cell stack can be configured so that non-reactive components tend to accumulate first in just one or a few fuel cells of the stack, rather than in the outlet region of each cell in the stack. The purge system may be controlled via a controller (see, for example, commonly assigned U.S. patent application Publication No. 2003/0022041, now U.S. Pat. No. 6,960,401).
However, although purging can improve performance of fuel cells having closed reactant supply systems, it wastes valuable fuel and increases the parasitic load on the system since purging equipment is required. Furthermore, the release of hydrogen into the ambient environment may be undesirable. Accordingly, there remains a need for improved methods of operating fuel cells having closed reactant supply systems for which purging is not necessary.